(c) The supply of an adequate amount of air, thoroughly distributed, at the point where the combustion of the hydrocarbons is to take place.
(d) The supply of fuel, and as far as possible, the manipulation of the fire, without opening of fire doors.
FIG. 23.—Chain-grate stoker.
FIG. 24.—Honey stoker.
These principles underlie the operation of smokeless furnaces and mechanical stokers. Stokers vary greatly in maintenance cost and in the associated cost of maintaining brickwork. In large plants, they save labor. A modern form of the old "Dutch oven" extension furnace, will give little smoke even when hand fired, if the furnace is not driven hard. The flames are kept hot, because they do not strike the boiler surface until combustion is complete.
Ira the chain grate stoker (see illustration) the coal is carried forward continuously on a moving chain, the ashes being dropped at the back end. The gases from the fresh fuel pass over the hotter coke fire on the back part of the grate. In underfeed